1. Field of the Invention
The present invention relates generally to image reading apparatuses and, more particularly to reducing the size of and simplifying the structure of a lens array in an image reading apparatus for illuminating an image with light from an illuminating light source, imaging light from the image onto a line sensor through the lens array and electrically reading image information.
2. Description of the Background Art
Office machines or communication apparatuses such as copiers or facsimiles include image reading apparatuses which optically read image information represented on the original of paper or drawings and convert the read information into an electric signal to be taken out. While there are various types of the image reading apparatuses, the image reading apparatus with a representative structure includes a light source, an optical lens and an image sensor. As to an operation of the image reading apparatus, in general, the apparatus illuminates the original having image information with white light from the light source, projects reflected light thereof being optically reduced in size onto a line sensor such as a CCD (Charge Coupled Device) image sensor through the optical lens, converts this projected light into an electrical signal and reads image information.
However, in an optical system employing a spherical lens as the optical lens, a spacing between the original and a light acceptance plane of image information is increased, resulting in an increase in the size of the apparatus. In order to avoid this increase in the size of the apparatus, a reflecting mirror for bending an optical path of light rays from an image is usually employed in the optical system to reduce the size of the apparatus. However, the provision of a device such as the reflecting mirror results in a complicated structure of the apparatus with a disadvantage that the apparatus is not effectively made compact.
Meanwhile, in recent years, due to practical application of a focusing lens for conducting light, an image reading apparatus, the size of which is reduced by integrally forming a reading optical system and an equal-scale magnification adhesion type reading line sensor together, has been in practical use. This image reading apparatus will now be described with reference to FIGS. 15 to 17.
Referring to FIG. 15, this image reading apparatus includes a light emitting diode 1 (not shown) to serve as a light source, a refractive index profile type lens array 2 constituting the optical system, and a line sensor 3 including a plurality of CCD image sensors for converting image information into an electric signal.
Next, referring to FIG. 16, the refractive index profile type lens array 2 includes a large number of focusing lens elements 2a of a cylindrical shape with a diameter of approximately 1 mm. These focusing lens elements 2a are interposed between frame plates 2b on both sides thereof. The frame plate 2b is formed of a glass fabric based epoxy resin black lamination plate (FRP) having almost the same thermal expansion property as the focusing lens element 2a so as to reduce thermal distortion and increase the strength thereof Black silicon rubber resin 2c is filled in spaces between the focusing lens elements 2a and the frame plate 2b. This silicon rubber resin 2c serves to remove flare light around the focusing lens elements 2a.
An operation of the image reading apparatus will now be described with reference to FIG. 17. Illumination light is first directed from the light emitting diode 1 to the surface of the original 4. Light from the image, reflecting on the surface of the original 4, is introduced into the refractive index profile type lens array 2 to be focused. This light is then transmitted through the refractive index profile type lens array 2 and is then imaged on the surface of the line sensor 3. The line sensor 3 converts this light from the image into an electrical signal to read image information.
A conjugate length TC of the optical system employing this refractive index profile type lens array 2 is expressed as TC=Z+2l.sub.o and is approximately 15-70 mm. This enables a distance between a plane of the original and a plane on which an image is read to be shorter compared to the conventional spherical lens, resulting in a smaller-scale image reading apparatus.
However, while the image reading apparatus employing the refractive index profile type lens array 2 can be reduced in size, it provides a disadvantage that the manufacturing cost of the apparatus is increased due to a difficulty in manufacturing the refractive index profile type lens array 2.
Some causes of the above disadvantage are described as follows
a The refractive index profile type lens array 2 includes three materials, the focusing lens elements 2a, the frame plate 2b and the silicon rubber resin 2c and thus is complicated to manufactured; and
b. All the focusing lens elements 2a are required to have uniform properties so as to have an equal refractive index profile constant and also an increased accuracy in the manufacture.
Further, the refractive index profile type lens array 2 also has another disadvantage that the removal of the flare light by the black silicon rubber resin 2c as a filler decreases an efficiency in utilizing an incident light.